The studies proposed use a multidisciplinary approach (i.e. anatomical, pharmacological, molecular and behavioral) to further our understanding of the cellular events that underlie neural plasticity in the CMS by investigating the roles that dopamine and BDNF play in the modulation of axonal sprouting and synapse replacement in the striatum. The rationale for the study is based on previous reports from our laboratory, as well as those of others, showing that: 1) axon terminals from contralateral corticostriatal neurons reinnervate denervated synaptic sites in the striatum after a unilateral cortex lesion; 2) synapse replacement involves the differential regulation of growth associated proteins and neurotrophins that are lesion-specific; and 3) depletion of striatal dopamine in combination with a unilateral cortex lesion results in an aberrant pattern of neurotrophin and growth associated protein gene expression and synapse replacement in the striatum that slows the recovery of motor function. The overarching hypotheses to be tested are: 1) that dopamine acts through specific subtypes of dopamine receptors to up-regulate candidate molecules known to participate in the regulation of neurite outgrowth and synaptogenesis after brain injury, and 2) that a key mediator in the compensatory response to a unilateral cortex lesion is the induction of BDNF in contralateral corticostriatal neurons. We will use the rat unilateral cortex lesion model and selective dopamine agonist and antagonist drug treatments to define the roles that specific subtypes of dopamine receptors play in the regulation of reactive synaptogenesis after brain injury. We will use intra-striatal infusions of the neurotrophin antagonist trkB-IgG to test the hypothesis that neurite outgrowth and synapse formation after the cortex lesion is mediated through BDNF and the trkB receptor. We will assess lesion- and treatment effect on: 1) upregulation of candidate molecules known to participate in the regulation of neurite outgrowth, synaptogenesis and cell survival after brain injury; and 2) synapse loss and replacement. Molecular and anatomical changes will be correlated with the recovery of sensorimotor function, as measured by the rotorod, forelimb-use asymmetry and forelimb placing tests. Data from our studies are fundamental for broadening our understanding of the capacity of the brain to compensate for injury, and central to the development of new treatment strategies that can translate the basic principles of neuroplasticity into effective clinical interventions. [unreadable] [unreadable]